This invention relates generally to electric propulsion systems for traction vehicles and, more particularly, to a method and apparatus for enhancing motive power output of electrically powered traction vehicles at high propulsion speeds.
An electric propulsion system for a traction vehicle, such as a large haulage truck, typically comprises a prime mover-driven electric generating means for supplying electric power to an electrical load comprising a pair of high-horsepower electric traction motors respectively connected in driving relationship to a pair of wheels on opposite sides of the vehicle. The prime mover is commonly a diesel engine, the generating means typically comprises a large alternator, the alternating current (AC) output terminals of which are connected to a power rectifying bridge, and the traction motors are generally adjustable speed, reversible direct current (DC) electric motors. A vehicle operator controls the vehicle speed and direction of travel, i.e., forward or reverse, by manipulation of a speed control pedal and a forward-reverse selector switch. This speed control pedal is adapted to control the engine speed (RPM) which determines the power output of the alternator, thus varying the magnitude of the voltage applied to the traction motors. In the propulsion or "motoring" mode of operation, the rotational speed of the motors (and hence the linear speed of the vehicle) is primarily dependent on both the magnitude of the alternator output voltage and the strength of the motor field. Separately excited motors are commonly used, with their field excitation being so controlled that the magnitude of motor field current is a predetermined function of the magnitude of armature current, thereby emulating the desirable characteristics of DC motors with series fields.
In the motoring mode of operation of a traction vehicle propulsion system, motor speed can vary over a wide range. Near the low end of the allowable speed range, the back EMF of each traction motor will be relatively low, armature current will be high, and therefore the motors can develop the high torque needed when a vehicle carrying a heavy payload is accelerating from rest or ascending a steep incline. Conversely, when motor speed is high (as will be true when a loaded vehicle is traveling on a nearly level roadway or when the vehicle is empty), back EMF will be high while load current (and hence torque) will be relatively low. Throughout the speed range, the magnitude of voltage that the alternator applies, via the rectifying bridge, to the armature windings of the motors will depend on the engine speed, the strength of the alternator field, and the magnitude of current that the alternator supplies to the armature windings, whereas the load current magnitude will depend on the motor torque demand.
Such a propulsion system characteristically has three different operating regions for any given engine speed: a current limit region at relatively low motor speeds, i.e., when the magnitude of load current is high; a constant horsepower (HP) region as motor speed varies through an intermediate portion of the full range of allowable speed variations; and a voltage limit region at speeds above the intermediate region and below maximum allowable speed, i.e., when load current magnitude is low.
Many interdependent variables are involved in sizing the various components of an electric propulsion system on board a large haulage truck. Because of weight and space constraints, it is not practical to use an alternator large enough to deliver to the traction motors, over the full range of allowable motor speeds (RPM) in the motoring mode of operation, 100% of the maximum rated power that the engine is capable of producing. Such a propulsion system is typically designed and controlled for 100% power utilization so long as the alternator output voltage is not higher than a certain magnitude (which is lower than the alternator's maximum allowable voltage) and for less than 100% power utilization otherwise. When the alternator voltage just equals such magnitude, the speed of each traction motor will be equal to a certain "corner point" speed. Corner point speed is the highest motor speed at which the alternator can safely supply the motors with a quantity of electrical power equivalent to the maximum rated mechanical power of the engine (less inherent losses). When motor speed exceeds this corner point speed, the alternator cannot deliver such equivalent power without the current in its field windings being increased above the maximum continuous field current rating of this machine. Corner point speed is lower than the maximum allowable motor speed which is established by physical or structural limitations in the traction motors. The physical sizes of the alternator and the motors are generally proportional to the product of the maximum rated power of the engine and the motor speed range over which maximum power can be continuously utilized without exceeding predetermined operating limits of the alternator and motors. For any given corner point speed and maximum allowable speed, the measure of performance for an electric propulsion system becomes the percentage of maximum rated power obtainable at maximum speed of the traction motors.
FIG. 1 illustrates an exemplary torque versus speed characteristic of a pair of serially connected DC traction motors, to which the rectified output voltage of an alternator is applied. The maximum allowable speed for this particular motor, determined by mechanical design limitations, is about 2320 shaft revolutions per minute. As motor speed varies in an intermediate portion of the full speed range, i.e., from about 300 RPM to about 1300 RPM, the propulsion system will operate in its constant power region to maintain motor horsepower at a desired constant limit which will be assumed equal to the maximum rated power available from the engine that is driving the alternator. This is illustrated in FIG. 1 by the solid-line curve between points O and A, it being understood that power is proportional to the product of speed and torque. Point A marks the corner point speed. To maintain the same power at higher motor speeds, current in the alternator field would have to exceed its maximum continuous rating. A phantom curve from point A to a point B illustrates a constant power continuation of the 0 to A curve for motor speeds between the corner point speed and the maximum allowable speed. In practice, however, the voltage limit region of operation becomes effective as motor speed increases above the corner point A, and motor horsepower will decrease instead of following a constant HP curve. This is illustrated by the solid-line curve between points A and C for a prior art propulsion system wherein the magnitude of alternator field current is reduced from its maximum rating in order to maintain the alternator output voltage at the magnitude it attained as motor speed increased to the corner point A. The vertical difference between the solid line from points A to C and the phantom line from points A to B represents an undesirably large reduction in motive power. To minimize the power reduction when the motor speed is above corner point speed and to utilize a higher percentage of maximum rated power at maximum allowable motor speed, a characteristic between these two lines, as illustrated in FIG. 1 by a broken-line curve between points A and D, would be desirable if it could be obtained without the alternator field current exceeding its maximum continuous rating and without motor armature current increasing above a predetermined commutation limit (which is proportional to the product of motor speed and armature current magnitude) and without violating either the maximum output voltage limit of the alternator or the minimum excitation limit of the motors (below which commutation degrades to an unacceptable level). These limits define the high speed range of operation of the conventional traction vehicle propulsion system. Power delivered to the traction motors at maximum motor speed would be optimized if alternator output voltage reached its maximum limit and motor armature current equaled its commutation limit concurrently with motor speed attaining its maximum allowable speed. The higher the power delivered to the motors at maximum speed, the steeper the grade that a loaded vehicle can ascend at a corresponding high speed, and consequently the greater the productivity of the vehicle.